1. Field of the Invention
This invention relates to compositions and methods for unsticking or freeing differential stuck pipe used in drilling a borehole for hydrocarbons in a subterranean formation.
2. Description of Relevant Art
Rotary drilling methods employing a drill bit and drill stems have long been used to drill wellbores in subterranean formation. Drilling fluids or muds are commonly circulated in the well during such drilling to cool and lubricate the drilling apparatus, lift cuttings out of the wellbore, and counterbalance the subterranean formation pressure encountered. The specific gravity of a well fluid is normally adjusted in such a way that the pressure of the fluid on the rock formations exceeds the formation pressure on the well. As a result, liquid components of the well fluid are forced into the formation surrounding the well while insoluble components settle on the walls of the well in the form of a stabilizing xe2x80x9cfilter cake.xe2x80x9d
Inevitably in drilling a well, the path followed by the drill pipes will deviate some from the vertical or other desired path for the well. At such consequent deviations or xe2x80x9ckinks,xe2x80x9d the rotating drill pipes may come into contact with the filter cake and even with the wall of the well and become buried therein, resulting in jamming or sticking.
Such jamming or sticking is particularly common when pipe connections, which are larger in diameter than the drill pipes themselves, come into contact with the kinks.
The most frequent cause of jamming or sticking of drill pipes is differential jamming. Differential jamming can occur when the rotation of the drill pipes is stopped. If a drill pipe then comes into contact with the filter cake adhering to the wall of the well, the drill pipe is forced into the wall of the well by the hydrostatic pressure of the column of mud. The contact surface is then isolated from the mud pressure. If, as is usually the case, the mud pressure is greater than the formation pressure, the contact surface is held fast to the wall by suction forces. The suction force increases with the thickness and compressibility of the filter cake. With time, even relatively large sections of the drill pipe can become held fast. Thus, prompt treatment is best.
Prompt freeing of the stuck drill pipe is also needed to alleviate lost drilling time and the consequent costs which that involves.
Spotting fluids are used by those skilled in the art to free stuck drill pipe. Spotting fluids need a good lubricating effect and the ability to ensure good oil wettability of the surfaces of the drill pipe and of the walls of wells coming into contact with the drill pipe.
To maximize utility in freeing stuck pipe, the spotting fluid should be applied at exactly the point in the well at which the pipe has become stuck. This is accomplished in the same general way in which the well fluid is introduced into the well. Instead of the well fluid, the spotting fluid is forced through the interior of the drill pipe to the bit, which leaves the bit, and slowly ascends in the well to the point of the stuck pipe. Thus, it is critically important to be able to exactly adjust the specific gravity of the spotting fluid so that it will be such as to keep the spotting fluid at that location in the well of the stuck pipe for sufficient time for its lubricity-enhancing properties to act to free the stuck pipe.
Since prompt use of a spotting fluid is important, it is standard practice at the drill site to store spotting fluid or spotting fluid concentrates which may rapidly be adjusted to the required density and consistency by addition of barite or other weighting materials and optionally other additives and water.
Spotting fluids known in the art typically consist of hydrocarbon mixtures, often based on diesel oils or mineral oils. Emulsifiers and surfactants are typical additives.
Such prior art spotting fluids may cause environmental concerns if they come into contact with ground soil formations. Further, since oil based spotting fluids mix with the drilling fluid during discharge from the well, such oil based spotting fluids contaminate the water based drilling fluids which then have to be discarded and separately treated to avoid environmental concerns, adding further to the drilling costs. The environmental concerns are enhanced in off-shore drilling where there is less space to store discarded fluids for treatment.
U.S. Pat. No. 4,964,615 issued in 1990 to Mueller et al. discloses an ester based spotting fluid that is said to have fewer environmental concerns than the mineral oil or diesel oil based fluids. However, a need continues to exist for more efficient and improved, environmentally friendly spotting fluids that are effective in freeing stuck pipe.
The composition and method of the present invention are useful in treating and alleviating stuck pipe, particularly differential stuck pipe, during drilling of a wellbore in a subterranean formation. The preferred composition of the invention is an invert oil mud system comprising iso-butyl oleate as the external phase and glycerin (preferably) or calcium chloride (CaCl2) solution as the internal phase. For application to stuck pipe downhole, this emulsion may be combined with, added to, or mixed with other fluids for carrying the emulsion to the treatment site and holding it there long enough to free the stuck pipe.